I. Field of the Invention
The invention is directed to an improved method of recovering oil from underground, relatively thick oil formations. More particularly, the invention is directed to the recovery of oil from such formations wherein a surfactant and a mobility control agent, alone or with additional optional agents, are injected into the formation at predetermined, vertically-spaced portions of the oil formation.
II. Description of the Prior Art
In the recovery of oil from subterranean formations, the oil is usually initially extracted by primary recovery techniques which rely on the naturally-occurring reservoir pressure forces to provide the pressure necessary to recover the oil. Once the primary recovery techniques are completed, a substantial amount of oil may be present in the reservoir, depending on the reservoir conditions. To recover this oil, secondary recovery techniques have been employed in the past. One of such secondary recovery techniques involves waterflooding, whereby water is injected into the porous rock, thereby displacing the brine and part of the oil in the portions of the formation contacted by the waterflood. As injected water invades the porous rock, an oil-water transition zone is created wherein the increasing saturation by the water causes the rupture of the continuous oil filaments. If the rock-oil-water system is preferentially water wet, the oil that remains is in the form of discontinuous globules or ganglia surrounded by rock and isolated by water. This oil is trapped by capillary forces and is normally referred to in the art as "residual oil". The residual oil communicates directly with water-saturated flow channels. It is known in the art that pores containing the residual oil contain very little water. The formation of the residual oil globules during the waterflooding process is dependent upon a number of criteria, such as oil-water interfacial tension and pore geometry. Tertiary oil recovery techniques have been developed in the past to recover this residual oil. In the tertiary oil recovery it is essential that the water in the water-saturated flow channels be replaced with a different fluid which interacts with the oil in the trapped ganglia and causes at least a part thereof to be displaced and coalesce with other similarly-displaced globules of oil to form a continuous oil stream which is forced to flow in the formation and then is recovered at the surface thereof, e.g., through a suitable well.
Various surfactants have been used in the past in the tertiary oil recovery techniques to cause the residual oil globules to be displaced and subsequently coalesce with other similarly-displaced oil globules to form a continuous oil flow, as described, e.g., by Reed et al, in "Some Physiochemical Aspects of Microemulsion Flooding: A Review", pages 383-437 of "Improved Recovery By Surfactant and Polymer Flooding", AMERICAN INSTITUTE OF CHEMICAL ENGINEERS SYMPOSIUM ON IMPROVED OIL RECOVERY BY SURFACTANT AND POLYMER FLOODING, Kansas City, Kans., 1976, Published by Academic Press, Inc., New York, 1977, the entire contents of which are incorporated herein by reference.
Petroleum sulfonates, usually obtained by subjecting hydrocarbons, such as crude oil or partially refined oil, to sulfonation conditions are known in the art as effective surface active agents used in enhanced oil recovery (EOR) applications. Various sulfonates and the use thereof as surface acting agents in the enhanced oil recovery applications are disclosed by Ahearn et al, U.S. Pat. No. 3,302,713, Patton, U.S. Pat. No. 3,373,808, Jones, U.S. Pat. No. 3,506,071, Gale, U.S. Pat. 3,861,466, and Aude et al, U.S. Pat. No. 4,290,973. Additionally, other materials known in the art can also be used as surfactants to free the oil and coalesce the dispersed globules thereof into a continuous stream. Such suitable other surfactants include, but are not limited to synthetic alkylaryl sulfonates, as described in U.S. Pat. No. 3,994,342; internal olefin and alpha-olefin sulfonates described in U.S. Pat. No. 4,549,607; propoxylated ethoxylated alcohol or alkylphenol ether sulfates and sulfonates described in U.S. Pat. No. 3,977,471; aliphatic and aromatic carboxylates described in U.S. Pat. No. 4,556,495; and nonionic surfactants, as described by Graciaa et al in the paper "Criteria for Structuring Surfactants to Maximize Solubilization of Oil and Water: Part 1-Commercial Nonionics", which appeared in Soc of Petr Eng Journal (Oct, 1982), 22, 743-749.
The entire contents of all of the above patents and publications are incorporated by reference herein.
The surfactants used in the large volumes necessary to recover a substantial proportion of the residual oil substantially increase the overall cost of oil recovery. Accordingly, it is important to minimize, when possible, the amount of the surfactant used to recover the residual oil from a particular formation. Similarly, mobility control polymers, also used in the past to control the flow of drive fluid and/or flooding fluids into the oil formation also add substantially to the overall cost of the oil recovery operation. Accordingly, it is also important to minimize the amount of such mobility control polymers used in the tertiary recovery operations.
Thus, a need still exists in the art for providing an improved process of recovering oil from underground formations wherein the amount of the surfactants and mobility control polymers used in the oil recovery process is substantially reduced.